In a typical continuous pulp digester, the wood chips and the white liquor are fed into the upper end of the digester, with the interior of the digester being maintained at a relatively high pressure (e.g. 200 PSI) and high temperature (e.g. approximately 380.degree. F). The mixture of chips and white liquor moves slowly and downwardly through the digester so that the total dwell time within the digester is generally between about two to four hours. During the period that the wood chips are in the digester, the white liquor reacts with the material in the wood chips to break down certain organic compounds in the wood chips so as to "delignify" the pulp.
At several locations along the length of the digester, portions of the liquid are extracted, either to be recirculated back into the digester, sent to an evaporator, or possibly to be processed elsewhere in the system. To retain the wood chips that are being processed in the digester, the liquid is extracted through sets of screens which are generally placed in sets at vertical locations circumferentially around the digester.
One of the problems in the operation of a digester is that the screens sometimes become clogged, possibly with wood chips and other material. Thus, it becomes necessary from time to time to clean the screens and remove the wood chips or other material which may have become lodged therein.
One prior art method of alleviating this problem to some extent has been to construct the screen with a circular flat surface and to place a rotating blade adjacent to the intake surface of the screen. The blade is rotated to create at the blade surface near the screen a suction, which dislodges wood chips which may be lodged in the openings in the screen. This is accomplished by mounting the screen in a circumferential ring formed in the wall of the digester in a form of a large diameter (e.g. 4 ft) nozzle, with the screen and the blade being mounted within this ring. Then a plate is mounted within the ring to close the ring opening to maintain the pressure and temperature conditions within the digester. There is positioned externally of the digester an electric motor which is used to drive the rotating blade. The shaft of the motor extends through the plate to connect the blade at the location inside of the digester. A rotating seal is maintained around the drive shaft of the electric motor at the location where it passes through the plate.
To the best knowledge of the applicant, while this cleaning apparatus has been used to some extent commercially, such commercial use has been rather limited, due to certain operating problems and other reasons. For example, there are difficulties in maintaining a seal where the shaft of the electric motor extends through the plate within the mounting ring, and leaks will sometimes develop. Another difficulty is the reinforcing of the main pressure vessel around these large diameter nozzles.